This invention relates generally to optical communications, and more particularly to optical telecommunication modules providing multiple modulated signals simultaneously from a single integrated device and receivers thereof.
Telecommunication systems can be broadly divided into two separate domains. In the line-side, or long haul side, data are transferred long distances between local hubs, with optical amplification compensating for loss and dispersion compensating modules maintaining signal integrity. On the client side, or datacom side, on the other hand, the distances are generally, short such that optical amplification and dispersion compensation is not needed.
Given the challenging requirements for transmitting signals thousands of miles, the components on the line side tends to be very high performance and quite costly, such as custom linecards or 300 pin transponders employing tunable lasers in the 1550 nm wavelength band and lithium niobate modulators for encoding data. Typically many closely spaced channels are multiplexed onto the same fiber (DWDM-dense wavelength division multiplexing). The channels are closely spaced at 25 GHz (0.2 nm), 50 GHz (0.4 nm) or 100 GHz (0.8 nm), and complicated optical filters are used to multiplex and demultiplex sometimes hundreds of these closely-spaced signals. The major advantage of DWDM in these systems is that optical amplifiers can boost the closely spaced channels simultaneously, eliminating the need for optical to electrical and back to optical conversion (OEO). OEO conversion may require demultiplexing hundreds of channels, receiving, reclocking, and regenerating in the electrical domain and then re-multiplexing them would be inordinately complicated and expensive
A very different school of thought has recently emerged that rather than using optical amplifiers together with dispersion compensation modules, attempts to make a very low cost OEO using integration. The idea is that low cost integration can eliminate the need for optical amplifiers and dispersion compensation by making OEO cost effective at every repeater. The system tries to use standard wavelength spacing and fully meet telecom's exacting standards. However, the integration compromises performance compared to discrete components, limiting chirp performance or degrading signal quality.
The client side, or the datacom side, is very different. In this case spans are generally short (40 Km or 80 Km maximum), and optical amplifiers are rare. Usually no dispersion compensation is used, and the reach is limited for example by the chirp of the signal and the dispersion of the fiber for solutions at 1550 nm wavelength, and limited by fiber loss for solutions at 1310 nm wavelength, with possibly the best performance obtained at 1550 nm. The components on the client side are usually lower cost, as exemplified by “pluggable” transceivers, with single channels passing through a fiber. Generally the lasers are uncooled for cost and power consumption reasons, and the resulting drifts in wavelength prohibit accurate wavelength multiplexing.
As bandwidth requirements increase, with different applications such as Internet games and IPTV (Internet protocol television) requiring large amounts of data, the bandwidth needs are increasing at all levels. In the long haul side, additional DWDM channels are possibly added as needed. Since this is a scalable procedure, costs do not rise dramatically. On the client or datacom side that is not multi-wavelength, generally the only way to increase capacity is to increase speed. However, this becomes increasingly impossible at higher speeds as one hits fundamental physical or engineering issues and simply increasing the speed of each link is not always a viable option. For example, CMOS drivers that work well at 2.5 Gb/s may produce insufficient current drive at 10 Gb/s. Similarly, lasers cannot be modulated at frequencies much past their internal relaxation oscillation resonance peak. The reach also drops as bandwidth increases, being roughly inversely proportional to the square of the bandwidth. A directly modulated laser, for example that can have a 150 Km reach at 2.5 Gb/s, may only go 10 Km at 10 Gb/s, the reach generally going inversely as the bandwidth squared. The use of electro-absorption modulators, or even lithium niobate modulators for reduced chirp only increases cost, complexity, and power consumption. Thus there is a need to increase the aggregate data rate for datacom applications without simply increasing the serial speed of the communications
The IEEE 802.3-2005 standard (which now incorporates the amendment 802.3ae-2002), incorporated by reference herein, relates to an architecture generally called the LX4 architecture. The LX4 architecture attempts to address the need for longer reach and higher speed transmission in the datacom arena by adding parallelism. The main problem the standard attempts to solve is that high bandwidth communication is limited in a multi-mode fiber, as modal dispersion limits the reach. Though 2.5 Gb/s signals can travel 300 m in multimode fiber, 10 Gb/s cannot. Therefore the standard calls for four separate channels to be multiplexed together at lower speeds to achieve a 10 Gb/s aggregate rate. Products on the market implement this standard with four separate lasers, each in an individual package and a bulk optic or fiber multiplexer at the transmitter side, and a demultiplexer and four separate photodetectors on the receiver side. Unfortunately, this results in increased component count, and therefore increased cost. Moreover, the effective transmission range of a system employing an LX4 architecture may be less than desired for many datacom applications. An integrated module that combines all these functionalities in a single chip would obviously reduce the cost of such a system.
In summary, integration of high performance components to reduce cost or eliminate optical amplifiers in the exacting world of telecom is challenging as integration compromises performance. However, the looser datacom world can allow for the compromises of integration if the manufacturing cost drops dramatically. This patent addresses the need for low cost integrated parallel components for datacom communications.